John,
There are probably as many storage options and methods that folks may have as there are readers here, but most have a few things in common. First, what do you mean by "portable"? Do you mean a somewhat trim case for external use, but requiring power from the wall socket, or do you mean truly portable, drawing power from the computer through its connecting bus (FW or USB)? If you want something to take along everywhere, then the bus-powered devices, which only use 2.5" drives (like those in laptop computers) are the smallest, but they can only use slower 5400rpm drives to get the 1TB option, or the faster 7200rpm drives that only go to 500GB capacity. The cases can have USB, FW400, FW800 or eSATA connections, or combinations, or all. Very versatile and very portable, but limited as mentioned.
Second, you mention for use with an older MacPro. All of the MacPro computers (desktop "tower" models) have FW800 ports, and some have FW400 ports, but they are totally compatible and only need a different cable to connect, plus the USB2 ports. If you are talking about a MacBook Pro (laptop), then you may have only FW800 + USB, or FW400 + USB, but again, just need different cables to use the FW with any case. If you are talking about an older MacBook laptop, not the aluminum 15" or 17" "Pro" models, then you may have USB only or USB + FW400 on some models. In any event, if you do have a FW400 port, you can still use FW800 accessories, but need a 6-pin (FW400) to 9-pin (FW800) cable to connect them. They are compatible, so no worry, but FW400 will be slower than the USB2 in many cases. So, a "portable" solution could be a case with a 500GB laptop drive inside and having "quad" connections (FW400/800, USB2 and eSATA), and you could connect that to virtually any Mac, desktop or laptop with the supplied cables. If you wanted a bigger storage device that sits on the desktop as an external drive, but needs power from the wall socket, you can get almost any type of case there also, again, with the FW400/800, and USB2 connections being the minimum you want, or getting one with all options including the eSATA.
As for brands and stuff, take a look at OWC (
http://eshop.macsales.com/shop/firewire/ ) to check out some of their case (external and portable) for single and multiple drives, and various port configurations, as well as being able to buy them as empty case or with drives already installed. They are a great starting place and have quality stuff at reasonable prices, plus very helpful tech staff. (I have no affiliation, but have tons of their stuff and have been using it for many years.)
As for archival storage....one of the easiest and maybe cheapest ways is to have another drive that is routinely updated just stored off-site. That means you have a main drive, a back-up, and another back-up that you store off-site. That way, if you lose the main and back-up in something like a fire or flood or they get stolen, you still have copies of everything stored someplace else, away from the main units. I am actually using bare hard drives for that purpose, since I have so many, and I just load them into a desktop device for transferring data (Voyager Q Desk Dock from OWC:
http://eshop.macsales.com/shop/NewerTech/Voyager/Hard_Drive_Dock ). That lets me only incur the added costs of just the drives, and not more cases and power units, etc.
Hope this is of some help, but I am sure there are lots of folks that will make other suggestions. Any simple drive in a case with at minimum a USB2 connection will do. There are some newer "standards" for faster USB3 and FW1600 and FW3200 being considered, but most will probably be back compatible to your FW400 and USB2 stuff, so not to worry. If you are looking for super speed and other performance things, there are even more options now and coming, but it sounds like you are just looking for more storage, some portability and archival needs only at this point.
LJ